Some parts are produced using additive manufacturing processes (e.g., three-dimensional printing) rather than traditional “subtractive” manufacturing process. Manufacturers employing additive processes desire effective and reliable testing methods to ensure quality control and to quantify the quality of parts. The quality and acceptability of additively manufactured parts may depend on various process parameters. For example, in selective laser melting (SLM) or electron beam melting (EBM), critical process parameters include powder characteristics and equipment setup parameters. In subtractive processes, material properties may be assessed based on samples of the bulk material from which the parts are fabricated. In additive processes, material properties may depend on the equipment setup parameters, such as scan speed and beam power. Therefore, the material properties of each additively manufactured part may be unique because density, microstructure, and mechanical properties are dependent on both powder characteristics and process parameters. Powder can be inspected and process parameters can be specified, but determining their cumulative effect on a part's material properties requires destructive testing of the part to confirm those properties. There are currently no suitable non-destructive tests to confirm the material properties of individual additively manufactured parts.
Resonant inspection techniques have been used to inspect parts produced by machining, casting, forging, and powered metallurgy processes. The parts are excited through direct contact, such as from impact hammers or piezoelectric actuators, and the response is measured with a microphone or with a direct contact piezoelectric actuator. The acceptability of an individual part is determined by comparing the peaks in a frequency spectrum of the response to those in a reference spectrum, wherein deviations in absolute frequency, relative frequencies, or peak amplitudes may be considered evidence of a defect. These techniques rely on contact with the parts being tested, either through impact by a hammer or contact with a piezoelectric transducer for excitation and response measurements. Such direct contact with the parts can directly affect the parts' responses and undermine the test results. Furthermore, because prior art processes input energy directly into the parts, the processes must be adjusted for the size and shape of each part being tested, which can lower efficiency.
Prior art testing methods rely solely on the frequency and amplitude of the response (or relative frequency and amplitude). As a result, they may be insufficiently sensitive to highly damped modes or modes with low radiation efficiency, which may undermine their ability to discriminate between acceptable and unacceptable parts. Also, prior art techniques use ultrasonic transducers which inherently have very little displacement capability, and therefore have very little power at low frequency. Ultrasonic transducers are only able to excite high frequency modes in parts and are not able to excite or detect low frequency modes, even though low frequency modes are likely to be most affected by the types of defects typically found in additively manufactured parts. Additionally, exciting and/or measuring parts through contact with ultrasonic transducers, which generally require acoustic coupling, adds to system damping.
This background discussion is intended to provide information related to the present invention which is not necessarily prior art.